149 lines
5.8 KiB
Groff
149 lines
5.8 KiB
Groff
.TH PCRECOMPAT 3
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH "DIFFERENCES BETWEEN PCRE AND PERL"
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.rs
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.sp
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This document describes the differences in the ways that PCRE and Perl handle
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regular expressions. The differences described here are mainly with respect to
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Perl 5.8, though PCRE versions 7.0 and later contain some features that are
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expected to be in the forthcoming Perl 5.10.
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.P
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1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
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it does have are given in the
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.\" HTML <a href="pcre.html#utf8support">
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.\" </a>
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section on UTF-8 support
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.\"
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in the main
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.\" HREF
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\fBpcre\fP
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.\"
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page.
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.P
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2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
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them, but they do not mean what you might think. For example, (?!a){3} does
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not assert that the next three characters are not "a". It just asserts that the
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next character is not "a" three times.
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.P
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3. Capturing subpatterns that occur inside negative lookahead assertions are
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counted, but their entries in the offsets vector are never set. Perl sets its
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numerical variables from any such patterns that are matched before the
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assertion fails to match something (thereby succeeding), but only if the
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negative lookahead assertion contains just one branch.
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.P
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4. Though binary zero characters are supported in the subject string, they are
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not allowed in a pattern string because it is passed as a normal C string,
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terminated by zero. The escape sequence \e0 can be used in the pattern to
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represent a binary zero.
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.P
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5. The following Perl escape sequences are not supported: \el, \eu, \eL,
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\eU, and \eN. In fact these are implemented by Perl's general string-handling
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and are not part of its pattern matching engine. If any of these are
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encountered by PCRE, an error is generated.
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.P
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6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
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built with Unicode character property support. The properties that can be
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tested with \ep and \eP are limited to the general category properties such as
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Lu and Nd, script names such as Greek or Han, and the derived properties Any
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and L&.
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.P
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7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
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between are treated as literals. This is slightly different from Perl in that $
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and @ are also handled as literals inside the quotes. In Perl, they cause
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variable interpolation (but of course PCRE does not have variables). Note the
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following examples:
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.sp
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Pattern PCRE matches Perl matches
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.sp
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.\" JOIN
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\eQabc$xyz\eE abc$xyz abc followed by the
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contents of $xyz
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\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
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\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
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.sp
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The \eQ...\eE sequence is recognized both inside and outside character classes.
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.P
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8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
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constructions. However, there is support for recursive patterns. This is not
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available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
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feature allows an external function to be called during pattern matching. See
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the
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.\" HREF
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\fBpcrecallout\fP
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.\"
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documentation for details.
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.P
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9. Subpatterns that are called recursively or as "subroutines" are always
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treated as atomic groups in PCRE. This is like Python, but unlike Perl.
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.P
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10. There are some differences that are concerned with the settings of captured
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strings when part of a pattern is repeated. For example, matching "aba" against
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the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
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.P
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11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),
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(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an
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argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing
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parentheses, PCRE does not set that capture group; this is different to Perl.
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.P
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12. PCRE provides some extensions to the Perl regular expression facilities.
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Perl 5.10 will include new features that are not in earlier versions, some of
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which (such as named parentheses) have been in PCRE for some time. This list is
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with respect to Perl 5.10:
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.sp
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(a) Although lookbehind assertions must match fixed length strings, each
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alternative branch of a lookbehind assertion can match a different length of
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string. Perl requires them all to have the same length.
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.sp
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(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
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meta-character matches only at the very end of the string.
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.sp
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(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
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meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
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(Perl can be made to issue a warning.)
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.sp
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(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
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inverted, that is, by default they are not greedy, but if followed by a
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question mark they are.
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.sp
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(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
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only at the first matching position in the subject string.
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.sp
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(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
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options for \fBpcre_exec()\fP have no Perl equivalents.
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.sp
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(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
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by the PCRE_BSR_ANYCRLF option.
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.sp
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(h) The callout facility is PCRE-specific.
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.sp
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(i) The partial matching facility is PCRE-specific.
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.sp
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(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
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different hosts that have the other endianness.
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.sp
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(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
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different way and is not Perl-compatible.
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.sp
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(l) PCRE recognizes some special sequences such as (*CR) at the start of
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a pattern that set overall options that cannot be changed within the pattern.
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge CB2 3QH, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 11 September 2007
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Copyright (c) 1997-2007 University of Cambridge.
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.fi
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